This invention relates generally to the field of colloidal chemistry and more particularly to reverse micelles and microemulsions.
Reverse (or inverted) micelles are small, dynamic aggregates of surfactant molecules surrounding a polar (typically aqueous) core dispersed in a nonpolar continuous (oil) phase. Reverse micelle solutions are clear and thermodynamically stable; as water is added to a reverse micelle solution a microemulsion is formed which contains nanometer-sized water droplets dispersed in a continuous oil phase. There is increasing interest in utilizing reverse micelle and microemulsion solutions for enhanced oil recovery (1,2), for the separation of proteins from aqueous solutions (3, 4, 8), as reaction media for catalytic (5, 23) or enzymatic (6) reactions and as mobile phases in chromatographic separations, and for polymerizations (5, 14).
The surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) forms reverse micelles in nonpolar fluids or oils without addition of a cosurfactant, and thus it is possible to study simple water/AOT/oil three component systems. To determine micelle (and, implicitly, microemulsion) structure and behavior in water/AOT/oil systems, investigators have studied a wide range of properties including conductivity (9), light (10), x-ray and neutron (11) scattering, and solution phase behavior (10). From information of this type one can begin to build both microscopic models and thermodynamic descriptions of these macroscopically homogeneous, but microscopically heterogeneous, micellar solutions.
Studies of reverse micelle solutions to date have been in liquids at temperatures well below the critical temperature (T.sub.c) of the continuous phase. For example, the critical temperature of iso-octane, which has been widely studied for AOT reverse micelles, is 288.degree. C. and the critical pressure (P.sub.c) is 45 bar. At moderate temperatures the low molecular weight hydrocarbons, such as ethane (T.sub.c =32.degree. C., P.sub.c =48 bar) and propane (T.sub.c =97.degree. C., P.sub.c =42 bar), can exist as supercritical fluids. For a pure component, the critical point represents the maximum temperature and pressure at which a two-phase single component system (liquid and vapor) can exist in equilibrium. In the supercritical fluid region, where temperature and pressure are above those at the critical point, the properties of the fluid are uniquely different from either the gas or liquid states (12, 13), but roughly variable with fluid pressure (or density) between the two limits. In particular, the solvating power of a supercritical fluid can be continuously varied over a wide range by adjusting fluid pressure. Additionally, the viscosities of supercritical fluids are typically 10 to 100 times higher (13) than gases but much less than those of liquids.
Researchers in this field are not known to have used supercritical or near critical fluids for the continuous phase of reverse micelles or microemulsions. Supercritical fluids have not been considered. Apparently, since supercritical fluids are dense gases, they have been overlooked or considered incapable of forming reverse micelle systems.